1. Field of the Invention
The present invention relates to a radio base station, a mobile communication system, and a broadcast information transmission method for performing packet communication by broadcasting.
2. Description of the Related Art
In mobile communication systems in recent years, only the broadcast transmission method is used as the transmission method when broadcast information is transmitted from a radio base station to a plurality of mobile stations.
Referring to FIG. 1, a mobile communication system is shown that is composed of radio base station 601 and mobile station 602. Radio base station 601 includes transmission buffer 603, broadcast encoder 604, and transmission section 605. Mobile station 602 includes: reception section 606, broadcast decoder 607, reception buffer 608, and host application 609.
Transmission buffer 603 temporarily stores broadcast information that has been transmitted from other nodes. Broadcast encoder 604 encodes broadcast information that has been temporarily stored in transmission buffer 603 for transmission to mobile station 602 by the broadcast transmission method. Transmission section 605 transmits to mobile station 602 while consuming the power necessary for transmitting the encoded broadcast information by the broadcast transmission method. Reception section 606 receives the broadcast information that has been transmitted to mobile station 602. Broadcast decoder 607 decodes the broadcast information that has been received by reception section 606. Reception buffer 608 temporarily stores the broadcast information that has been decoded. Host application 609 processes the broadcast information that has been temporarily stored in reception buffer 608.
However, when there is a mobile station among the plurality of mobile stations that are the objects of transmission that is, for example, positioned in a location that is remote from the radio base station and in which reception quality is poor, such as a location close to the cell border, transmitting broadcast information by the broadcast transmission method results in an increase in the total transmission power and a deterioration of transmission efficiency.
As a response to this problem, means are provided for transmitting broadcast information by the unicast transmission method that uses individual channels for each of the plurality of mobile stations that are the objects of transmission. The transmission method is then selected depending on which method has the lowest total transmission power: transmission by the broadcast transmission method or transmission by the unicast transmission method that uses individual channels. A method in which transmission is realized by using this transmission method to reduce the total transmission power is disclosed in JP-A-2004-193888. In this case, the total transmission power is the sum of the power for transmitting data to each of the plurality of mobile stations that are the objects of transmission.
Further, when broadcast information is transmitted from a radio base station to a mobile station by the broadcast transmission method and an error occurs in the data that are received at the mobile station, a request to resend the data in which the error occurred is submitted to the radio base station using an individual channel. The radio base station, having received the request to resend, then separately resends the data in which the error occurred to the mobile station by the unicast transmission method that uses individual channels. A method for thus improving transmission efficiency is disclosed in JP-A-2004-12856.
However, in the method disclosed in the above-described JP-A-2004-193888, the problem arises that, when data are transmitted to each mobile station by the broadcast transmission method and errors occur in data that are received in a portion of the mobile stations among the mobile stations that receive data transmitted from the radio base station, requests for resending the data in which errors occurred are not submitted.
In addition, in the method that was disclosed in the above-described JP-A-2004-128563, no consideration is given to increasing the efficiency of the total transmission power for transmitting data.